Voice coders, also termed vocoders, are circuits that reduce bandwidth occupied by voice signals, such as by using speech compression technology, and replace voice signals with electronically synthesized impulses. For example, in some vocoders an electronic speech analyzer or synthesizer converts a speech waveform to several simultaneous analog signals. An electronic speech synthesizer can produce artificial sounds in accordance with analog control signals. A speech analyzer can convert analog waveforms to narrow band digital signals. Using some of this technology, a vocoder can be used in conjunction with a key generator and modulator/demodulator device to transmit digitally encrypted speech signals over a normal narrow band voice communication channel. As a result, the bandwidth requirements for transmitting digitized speech signals are reduced.
A new military standard vocoder (MIL-STD-3005) algorithm is referred to as the Mixed Excitation Linear Prediction (MELP), which operates at 2.4 Kbps. When a vocoder is operated using this algorithm, it has good voice quality under benign error channels. When the vocoder is subjected to a HF channel with typical power output of a ManPack Radio (MPR), however, the vocoder speech quality is degraded. It has been found that a 600 bps vocoder provides a significant increase in secure voice availability relative to the 2.4 Kbps vocoder.
A need exists for a low rate speech vocoder with the same or better speech quality and intelligibility as compared to that of a typical 2.4 Kbps Linear Predictive Coding (LPC10e) based system. A MELP speech vocoder at 600 bps would take advantage of robust and lower bit-rate waveforms than the current 2.4 Kbps LPC10e standard, and also benefit from better speech quality of the MELP vocoder parametric model. Tactical ManPack Radios (MPR) typically require lower bit-rate waveforms to ensure 24-hour connectivity using digital voice. Once HF users receive reliable, good quality digital voice, wide acceptance will provide for better security by all users. An HF user will also benefit from the inherent digital squelch of digital voice and the elimination of atmospheric noise in the receive audio.
Current 2.4 Kbps vocoders using the LPC10e standard have been widely used within encrypted voice systems on HF channels. A 2.4 Kbps system, however, allows for communication on narrow-band HF channels with only limited success. A typical 3 kHz channel requires a relatively high signal-to-noise ratio (SNR) to allow reliable secure communications at the standard 2.4 Kbps bit rate. Even use of MIL-STD-188-110B waveforms at 2400 bps would still require a 3 kHz SNR of more than +12 dB to provide a usable communication link over a typical fading channel.
While HF channels typically permit a 2400 bps channel using LPC10e to be relatively error free, the voice quality is still marginal. Speech intelligibility and acceptability of these systems are limited to the amount of background noise level at the microphone. The intelligibility is further degraded by the low-end frequency response of communications handsets, such as the military H-250. The MELP speech model has an integrated noise pre-processor that improves sensitivity in the vocoder to both background noise and low-end frequency roll-off. The 600 bps MELP vocoder would benefit from this type of noise pre-processor and the improved low-end frequency insensitivity of the MELP model.
In some systems vocoders are cascaded, which degrades the speech intelligibility. A few cascades can reduce intelligibility below usable levels, for example, RF 6010 standards. Transcoding between cascades greatly reduces the intelligibility loss in which digital methods are used instead of analog. Transcoding between vocoders with different frame rates and technology has been found difficult, however. There are also known systems that transcode between “like” vocoders to change bit rates. One prior art proposal has created transcoding between LPC10 and MELPe. A source code can also provide MELP transcoding between MELP1200 and 2400 systems.